Such control methods serve to actuate injection valves in such a way that they are opened and closed again as exactly as possible at predefined times in order to inject as precisely as possible a predefined quantity of a pressurized fuel into the internal combustion engine. In this way, and, if appropriate, also by means of additional pre-injections and/or post-injections in addition to a main injection within one injection cycle, the efficiency of the internal combustion engine can be increased and at the same time exhaust gas emissions and noise emissions can be reduced.
An injection valve, frequently also referred to as injector, has a closing element which can be moved to open and close the injector by means of an actuator drive, referred to below in short as a drive. In the closed state of the injector, in which no injection takes place, the closing element is in a closed position, also referred to as a closing position, in which it closes off all the injection openings of the injector. The closing element can be raised starting from a closed position by means of the drive in order in this way to clear at least some of the injection openings and trigger the injection.
The closing element frequently has a nozzle needle or is configured as such. In its closing position, this nozzle needle then typically sits on what is referred to as a needle seat of the injector. The drive of the injector comprises, for the purpose of moving the closing element, an actuator which is typically configured to raise the closing element from the closed position to a stroke height as a function of a control signal, to hold said closing element at this stroke height and/or to move the closing element back into the closed position again. This actuator can, for example, be formed by a piezoelement which expands or contracts owing to electrical charging or discharging processes and in this way triggers a lifting or closing movement of the closing element. Such actuators which are also referred to as piezoactuators are particularly well suited for precise and delay-free movement of the closing element. This is the case in particular with what are referred to as directly driven (piezo) injectors in which direct and delay-free transmission of force between the piezoactuator and the closing element is made possible. Such a directly driven injector is known, for example from document EP 1 760 305 A1, the disclosure content of which is hereby completely incorporated.
When known control methods are used for injection valves in which injection valves of an injection system, for example of a common rail injection system, are actuated by means of control signals, it frequently proves problematic that identical injection valves within the injection system can differ in terms of their response behavior to the control signals. Differences in the opening behavior of the injection valves prove particularly problematic since they can have particularly strong effects on the quantity and time of injections.
Such differences can be caused, for example, by fabrication tolerances, progressive wear phenomena or other, possibly also time-dependent, interference influences. As a result of wear it is possible, for example, for play which becomes greater with time to come about between the piezoactuator and the closing element, with the result that the injection valve does not open with a precisely known deceleration until after the injection valve has been actuated. Document DE 10 2008 023 373 A1 describes, for example, a control method with which an idle stroke, which comes about owing to the abovementioned play between the piezoactuator and the closing element, can be taken into account and compensated. In this case the position of a maximum of a force profile of a force applied to the closing element by the piezoactuator is determined, and the time of the actual opening of the injection valve is inferred on the basis of the chronological position of this maximum. The position of the maximum of the force profile is subsequently used as a controlled variable of the control method and adjusted to a setpoint value.
In addition to the described idle stroke, the response behavior of the injector to the actuating signal can also depend on many further influencing factors and interference variables such as, for example, wear of further components, a nozzle body temperature, a fuel viscosity, a fuel pressure, a temperature of the piezoelement of the drive, as well as a prehistory of the piezoelement.
In addition to the most precise possible setting of the injection time and of the injection quantity, setting of an injection rate is also increasingly required. In this context, the fuel quantity injected per time unit will be referred to as the injection rate. These requirements demand a high control quantity of the control method, which also then has to be sufficient if the response behavior and, in particular, the opening behavior of an injector which is used changes over time and/or deviates from an expected behavior or a reference behavior of injectors of the same design. In particular, owing to deviating opening behaviors of individual injectors of an injection system it proves particularly difficult to achieve identical injection rates with the individual injectors of the system (identical setting).